Hearing aid with beam forming properties

ABSTRACT

A hearing aid with beam forming properties, having at least two microphone channels ( 1   a   , 1   b ) for at least two microphones ( 2   a   , 2   b ), the microphone channels each comprising an analog to digital converter ( 3   a   , 3   b ) and having at least one programmable or program controlled signal processor ( 5 ), as well as a digital to analog converter, and at least one receiver and a battery for power supply. The hearing aid particularly comprises in each microphone channel ( 1   a   , 1   b ) a sigma-delta-type analog to digital converter ( 3   a   , 3   b ) including a digital low pas filter and a decimator  94 ) for converting a 1 Bit Stream of a high clock frequency into a digital word sequence of a lower clock frequency. At least one of the at least two microphone channels contains a controllable delay device ( 6 ) connected to the input side of the respective digital low pass filter and decimator ( 4 ) of the channel, the delay device ( 6 ) being controllable by the at least one signal processor ( 5 ). The delay device is preferably integrated into the sigma-delta-ADC ( 3 ).

BACKGROUND OF THE INVENTION

In the EP 0820210 A2 a method and apparatus for beam forming of themicrophone characteristic has been disclosed, by which a pre-determinedcharacteristic of amplification in dependency of the direction fromwhich acoustical signals are received at two spaced apart microphones isformed in that repetitevely a mutual delay signal is determined from theoutput signals of the microphones and according to the reception delayof the microphones, one of the output signals is filtered, thereby thefiltering transfer characteristic is controlled in dependency of themutual delay signal. The output signal of the filtering is exploited aselectrical reception signal.

Thus, in principle the time delay or phase lag between the two outputsignals of the two microphones is used for a beam forming operation.

In a digital hearing aid the single samples are taken with a timedifference equally divided by the sampling frequency, f.i. normally 32usec. The desired delay between two or more microphone signals aretypically less than 32 usec, e.g. 15 usec. A way to obtain a delay whichis less than one sample is to have the DSP interpolate signal valuesbetween two samples with a certain delay and use those delayed samplevalues in the further processing. But this requires many calculationsand takes up valuable space and power in the DSP.

Also, the signal will be somewhat distorted as the delyed samples arenot “true” samples.

However, for an active control of beam forming properties in adirectional hearing aid, the delays that could be realized, based on thesample frequency and conventional shift register technology would bemuch too long to be useful.

In order to realize sample delays as low as 1 usec the conventionaltechnology can not be used.

Thus, it is an object of the present invention to create a novel hearingaid with beam forming properties in which an active control of the delayof at least one of the incoming signals of a hearing aid having at leasttwo microphones can be used for active beam forming. With such a hearingaid a great number of various directional orientations of hearing aidscould actively and controllably be realized.

Particularly, by using faster sampling rates, the samples because oftheir shorter time intervals could be used directly, so that desirableshort delays could be realized.

By using a sigma-delta converter with a sampling rate or clock frequencyof f.i. 1 MHz and by inserting a 1 bit adjustable and controllabledigital delay line in the bit stream from one of the sigma-deltaconverters to the corresponding decimator filter of the converter onecould obtain delayed difference steps of multiples of 1 usec, whichcould not be achieved with conventional hearing aid technology.

In connection with a single channel hearing aid, it is known per se fromGerman published patent application No. 44 41 996 to use ahigh-frequency-clocked signal-delta converter as part of an advantageousamplifier stage.

SUMMARY OF THE INVENTION

For this purpose a new hearing aid with beam forming properties has beendeveloped, which has at least two microphone channels for at least twomicrophones, said microphone channels containing each an analog todigital converter, and having at least one programmable or programmeddigital signal processor, as well as a digital to anlalog converter, atleast one receiver and a battery for power supply.

This new hearing aid, in accordance with the present invention, containsin each of said microphone channels a sigma-delta-type analog to digitalconverter including a digital low pass filter and decimator filter forconverting a 1 bit stream of a high clock frequency into a digital wordsequence of a lower clock frequency, whereby at least one of said atleast two microphone channels contains a controllable delay deviceconnected to the input side of the respective digital low pass filterand decimator filter of said channel, said delay device beingcontrollable by said at least one digital signal processor.

It is advantagous to have said delay device integrated into thesigma-delta ADC.

It is of particular importance to use, as a delay device, a programmableor program controlled tapped shift register for realizing variousdifferent delays of the bit stream signals before their entering therespective digital low pass filter and decimator. In order to realizecontrollable delays as short as 1 usec it is of advantage to use a clockfrequency for the sigma delta ADC in the range of 1 MHz or even higherand a clock frequency in the area of 10 to 50 kHz for the digital lowpass filter and decimator filter.

It is now obvious that with such a configuration of the input side of abeam forming hearing aid with active beam control various additionalpossibilities exist which are subject of the remaining claims.Particularly, by this new hearing aid a very high resolution delay maybe achieved.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will now be described in more detail in conjunction withseveral embodiments and the accompanying drawings:

In the drawings

FIG. 1 shows schematically a number of polar diagrams of variations ofbeam directions which could be realized by the present invention;

FIG. 2 shows schematically the general structure of a sigma-delta analogto digital converter (ADC);

FIG. 3 shows schematically a first embodiment of the invention;

FIGS. 4, 5, 6 and 7 schow schematically further embodiments of theinvention.

FIG. 1 illustrates four different directional patterns in polardiagrams.

FIG. 1a represents the hypercardioid system which has a very desirabledirectional effect. 1 b is the bidirectional. System which has no delayfor any of the two microphones and therefore attenuates all soundscoming directly from the sides (90 degrees and 270 degrees) as the twomicrophones level out each other. 1 c is the cardioid which must have adelay in the front microphone equal to the longitudinal delay betweenthe inlet ports of the two microphones. Finally, 1 d is theomnidirectional or spherical system, which is simply a single microphone(the other microphone is switched off), or the two microphone signalsare added and not subtracted from each other.

However, by controlling the various delay devices, other directionalpatterns could be realized. This will be more evident from the followingdescription of the FIGS. 2 to 7.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS OF THE INVENTION

As has been explained above, for realizing hearing aids in accordancewith the present invention, normal analog to digital convertersoperating with clock frequencies of 16 or 32 kHz could not be used forrealizing delays in the range of 1 usec or multiples thereof.

FIG. 2 shows a well known type of a first order sigma-delta digital toanalog converter comprising basically a summing circuit, an integrator,a comparator stage (1 bit ADC) and a digital low pass filter 4 and adecimator filter. The comparator stage is controlled by a high frequencyclock generator supplying clock pulses in the aerea of 1 MHz or higher.The output of the integrator is connected also to a 1 bit DAC, theoutput of which is connected to a second input of the summing circuit.The digital low pass filter and decimator filter operates at a clockfrequency of f.i. 32 kHz and converts the 1 bit stream of a clockfrequency of about 1 MHz into a sequence of data words at the lowerfrequency, f.i. 16 or 32 kHz. These data words could e.g. be 20 bitwide. These data words are then, normally, applied to a programmable orprogram controlled digital signal processor.

It is to be understood that all embodiments of the invention will makeuse of such sigma-delta-type ADC's, provided a high clock frequency inthe area of 1 MHz or higher is used for controlling the comparator.

FIG. 3 shows, schematically, a first example of the inventive conceptualdesign.

Two microphone channels 1 a and 1 b comprise microphones 2 a and 2 b andsigma-delta analog to digital converters 3 a, 3 b including digital lowpass filters and decimator filters 4 a and 4 b for supplying data wordsto a programmable or program controlled digital signal processor 5.

In one of the microphone channels a controllable delay device 6 isincluded. This delay device is typically a multiple tap shift registerand the control signal coming from the DSP 5 will decide how many 1 bitstages each sample of the bit stream will go through (and thus bedelayed by) before they are tapped and sent furtheron in the system, inthis case to the digital low pass filter and decimator 4. The resultingdelay is equal to the number of stages times the inverse sampling rate,f.i. 1 MHz.

With this high resolution of the sigma-delta ADC the time resolution canbe 30-40 times higher than would be possible inside the DSP using itsclock as a basis for delays. Normally, this setup can only handle beamforming from the front or from the back but not both. The controllabledelay would be controlled by the DSP so that the DSP direct the beam inthe desired directions.

FIG. 4 shows a further embodiment of the invention. All parts andcomponents which are the same as in FIG. 3 are designated with the samereference numerals and need not to be described again. This holds truefor all other FIGS. as well so that only the differences will beexplained in detail.

In FIG. 4 both microphone channels 1 a and 1 b contain each acontrollable delay device 6 a, 6 b. They can, of course, be controlledindependently and separately. Although two delay devices areincluded,only one of the two may be controlled whereas the other isswitched off.

The output signals of the digital low pass filter and decimator filters4 a and 4 b are combined in a summing circuit 7 and passed on to theDSP. Thus, by having controllable delay in both sigma-delta convertersit will be possible to reverse the beam forming operation and use itboth at front and back.

In FIG. 5, which in almost all respects is similar to FIG. 4, the outputsignal of the lower one of the two microphone channels 1 b is nowconnected to a first input of a multiplier stage 8, the second input ofwhich receives a controlling input from the DSP.

The output of the multiplier stage 8 is applied to the second input ofthe summing circuit 7, which feeds into the DSP.

It may be desirable to make a shift from e.g. the hypercardiodid to theomnidirectional characteristic. For this purpose the multiplier 8 isadded after the digital low pass filter and decimator filter for onemicrophone or for both. The DSP then can multiply the samples withfactors between −1 and +1.

FIG. 6 shows the extension from two microphone channels to multiplemicrophone channels. Again, controllable delay devices may be arrangedin one channel, in two channels or in all channels. The output signalsof all channels are combined in a combination circuit 9, the outputsignals of which are applied to the DSP. This combination could beeffected with different factors between −1 to +1, if convenient.

FIG. 7 finally, shows another variation of the inventive circuit inwhich at least one of the microphone channels has not only one delaydevice and one digital low pass filter and decimator filter but two ofthose in parallel. It is also conceivable to have these parallelarrangements in one or more channels, even in all of them.

It is also possible to use more than two delay devices in parallel in atleast one of said microphone channels, all connected to their respectivedigital low pass filter and decimator filter of said at least one ofsaid channels.

What is claimed is:
 1. Hearing aid with beam forming properties,comprising at least two microphone channels (1 a, 1 b) for at least twomicrophones (2 a, 2 b), a digital programmable or program controlledsignal processor (6), a receiver and a battery for power supply, whereineach microphone channel (1 a, 1 b) contains a sigma-delta-type analog todigital converter (3 a, 3 b), a digital low pass filter and decimator(4) for converting a 1 Bit stream of a high clock frequency of saidconverter into a digital word sequence of a lower clock frequency, andat least one of said at least two microphone channels contains acontrollable delay device (6) connected to the input side of therespective digital low pass filter and decimator (4) of said channel,said delay device (6) being controllable by said processor (5). 2.Hearing aid in accordance with claim 1, wherein the delay device (6) isintegrated into said sigma-delta-converter (3).
 3. Hearing aid inaccordance with claim 1, wherein a first order sigma-delta converter isused in said at least two microphone channels.
 4. Hearing aid inaccordance with claim 1, wherein a second order or even higher ordersigma-delta-converter is used in said at least two microphone channels.5. Hearing aid in accordance with claim 1, wherein the clock frequencyfor the sigma-delta-converter (3) is in the range of 1 MHz or higher andthat said lower clock frequency for the digital word sequence is in therange of 10 to 50 kHz.
 6. Hearing aid in accordance with claim 1 whereinsaid delay device comprises a programmable or program controlled tappedshift register for realizing various different delays of said one bitstream before entering said digital low pass filter and decimator. 7.Hearing aid in accordance with claim 1, wherein the output signals ofsaid at least two microphone channels are combined in a summing circuit(7) for controlling said processor.
 8. Hearing aid in accordance withclaim 1, wherein in each said sigma-delta converter (3 a, 3 b) of saidat least two microphone channels (1 a, 1 b) the controllable delaydevice (6 a, 6 b) is included.
 9. Hearing aid in accordance with claim1, wherein one of said at least two microphone channels is directlyconnected to the summing circuit (7), whereas the other of said at leasttwo microphone channels is connected to a first input of a multiplierstage (8), the output of which is coupled to said summing circuit (7),whereas a second input of said multiplier stage (8) is controlled by thedigital signal processor (5).
 10. Hearing aid in accordance with claim1, wherein each of said at least two microphone channels being equippedwith sigma-delta analog to digital converters (3) including at least insome of them said controllable delay devices, the outputs of which arecombined in a combination circuit, or an integrator circuit, saidcircuit is (9) connected to the input side of said processor (5). 11.Hearing aid in accordance with claim 1, wherein said at least one of thesaid at least two microphone channels is equipped with a sigma-deltaanalog to digital converter including at least two delay devices inparallel operating on two digital low pass filters and decimators, theoutput signals of all said digital low pass filters and decimators arebeing combined in a combination circuit connected to the input side ofsaid at least one digital signal processor.
 12. Hearing aid inaccordance with claim 1, wherein a remote control unit for controllingthe said digital signal processor for effecting various beam formingdirectional orientations of said at least two microphones by influencingone or more of said delay devices for introducing various differentdelays.